KVM: MMU: clear apfs if page state is changed

If CR0.PG is changed, the page fault cann't be avoid when the prefault address
is accessed later

And it also fix a bug: it can retry a page enabled #PF in page disabled context
if mmu is shadow page

This idear is from Gleb Natapov

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>